THE URO-GENITAL SYSTEM 



375 



activity in any adult fish or amphibian. Originally involving 

 a large number of somites the mesonephros extends, usually as 

 a pair of long, narrow organs, along a large portion of the 

 trunk, lying close up against the vertebrae and ribs. The 

 original nephridia become greatly multiplied and lose more 

 or less of their original segmental arrangement, the nephro- 

 stomes appearing irregularly along the ventral surface, that is, 

 the surface turned toward the ccelomic cavity. The mesone- 



FIG. 108. Diagrams to illustrate a theory of the relationship of the 

 pro- and meso-nephrotic tubules to each other. [After GEGENBAUR.] 



(A) Stage of the pronephros. Its nephrostome (p) is placed opposite the 

 glomerulus, but with a small portion of ccelom interposed between them. (B) Stage 

 of the mesonephros. Here by the formation of a new nephrostome at s the inter- 

 posed piece of the ccelom has become included in the nephridium, forming a Bow- 

 man's capsule. 



phrotic (Wolffian) duct lies along its outer side and sustains 

 that relationship to the cloaca noted above under the pro- 

 nephrotic system, usually opening by a urinary papilla into 

 the dorsal wall of the cloaca. 



The third urinary system, the metane phrotic, arises di- 

 rectly from the second, and thus the relation between the two 

 is closer than that between the second and the first. It is the 

 definitive urinary system of the amniotes, and in all reptiles, 

 birds and mammals ultimately replaces the mesonephrotic, 

 although this latter system is well developed during embry- 



